add encoder/decoder api test for LTR and EC flag

review at: https://rbcommons.com/s/OpenH264/r/805/
1. add encoder-engine-decoder API UT for: IDR request, LTR request
2. add decoder api UT for EC: Disable/Enable EC
3. Fix a decoder bug when EC is disabled
   itoltalMbDec should be set to zero when encouter a new sequence/picture, even   when EC is disabled.